I had an absolute nightmare dealing with these agents. To start with, I had newly moved to UK and was staying in a hotel. It was necessary for me and my wife to move to the flat chosen soon as we were paying hefty hotel bills. In spite of making this clear with Dickinson Harrison, their process was slow and there were unnecessary delays after producing all documentation. When questioned they only said their systems were down.
Maintenance: If something breaks, you are in for a shock. Even after 20 days, I am waiting for them to fix the hot water trouble. Yes, no hot water in peak winter - the broiler broke and someone from their side suggested my wife to use the "kettle" meanwhile. When I suggested I get it fixed myself and they pay the bill, they said "you can't do that - you will be breaking the tenancy agreement". It seems like they have hundreds of rules for tenants but absolutely don't have any for themselves. Why should someone pay full rent only to get broken services.
I have had couple of other experiences especially related to maintenance, written to them - either you get no reply or acknowledgement or when you call them, they will just say maintenance is on it. It takes them week to do something about it. Avoid if you can. Being a foreigner and wanting a flat urgently, I didn't have much choice.